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Two Islands, Two Wet Sides: Wind Belts and Landslides

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Two Islands, Two Wet Sides: Wind Belts and Landslides

Use a spinning-disk model and island records to explain which mountain slope gets the rain.

The spinning-disk model

Read the passage and the disk data, then answer the questions.

Why One Side of the Island Slides

1Ridgeback is a mountain town on Kestrel Island, a large island at about 45° north latitude (45°N). A steep ridge runs from north to south down the middle of the island, with open ocean on both sides. The town's hazard office, run by Mr. Bertrand Okoye, keeps records of landslides and earthquakes. Sixth graders Odette Laurent and Kwame Boateng, who volunteer at the office, noticed that the towns on the west slope have many more landslides than the towns on the east slope. They compared Kestrel Island with Palmetto Island, which has the same kind of north–south ridge but lies at about 18°N. The islands and their numbers are made up, but the patterns are based on real places.

2Mr. Okoye explained that most landslides on both islands happen during or after long, heavy rains, when soaked soil grows heavy and slips. A smaller number are set off by earthquakes.

3The students started with a model of how air moves around Earth. Sunlight strikes Earth most directly near the equator, so the air there is heated the most. This warm air rises, spreads toward the poles high in the atmosphere, cools and sinks back to the surface at about 30°N and 30°S. Near the surface, part of the sinking air flows back toward the equator. Between about 30° and 60° latitude, surface air flows the other way, away from the equator and toward the pole.

4To see how Earth's spin changes the path of moving air, the students built a second model. They taped a paper disk with a radius of 20 cm onto a turntable. While Kwame turned the disk slowly, Odette slid a marker along a ruler that did not move, from the center of the disk toward a dot at its edge, taking about 1 second for each line. A counterclockwise spin stands for Earth as seen from above the North Pole, and a clockwise spin stands for Earth as seen from above the South Pole. For each line, they measured how far its end landed from the dot and noted which side of the dot it was on, looking from the center toward the dot. Each result is the average of 3 trials.

5Air over the ocean picks up water vapor. When the wind pushes moist air up a mountain slope, the air cools, and much of its water vapor condenses into clouds and rain. By the time the air crosses the ridge and sinks down the other side, it has lost much of its water.

Table 1. Spinning-Disk Model (average of 3 trials)
Spin directionTurns per minuteLine ended from dot (cm)Side of the dot
No spin00On the dot
Counterclockwise24Right
Counterclockwise48Right
Counterclockwise613Right
Clockwise48Left
1.
Use Table 1.

With a counterclockwise spin, the line ended cm farther from the dot at 6 turns per minute than at 2 turns per minute.

2.
Which conclusion does Table 1 support best?
Question 2 options
3.
Use paragraph 4. What does the clockwise trial stand for in the model?
Question 3 options
4.
Use paragraph 3 and the disk model. A wind is named for the direction it blows from: a wind moving toward the south is a wind from the north. Match each place to the surface wind it most likely has.

Rain, landslides and earthquakes on two islands

Use Table 2, the passage and your wind answers.

Table 2. Island Towns (averages for 1995–2024)
TownIslandSide of ridgeRain (mm/yr)Landslides/10 yrFelt quakes/10 yr
RidgebackKestrel (45°N)West2,400156
AshfordKestrel (45°N)East60037
Saltpan CovePalmetto (18°N)West50024
CloudfallPalmetto (18°N)East2,500163
Landslides/10 yr counts every landslide the office recorded near the town, whatever set it off. Felt quakes/10 yr counts earthquakes strong enough for people in the town to feel.
5.

Ridgeback gets times as much rain each year as Ashford.

6.
Which statement best describes Table 2?
Question 6 options
7.
Which explanation of Ridgeback's heavy rain fits the models best?
Question 7 options
8.
Kwame wonders whether earthquakes explain why Ridgeback has five times as many landslides as Ashford. What do the data in Table 2 suggest?
Question 8 options
9.
Underline each fact from Table 2 that is evidence that heavy rain, not earthquakes, explains most of the landslides.

10.
Each slope below is on a large island with a north–south ridge and ocean on both sides, like Kestrel and Palmetto. Use the wind belts to sort each slope.
11.
A visitor asks Mr. Okoye, "Why is the landslide side of the ridge the west slope on Kestrel Island but the east slope on Palmetto Island?" Write a claim, use data from both tables as evidence, and explain your reasoning using uneven heating and Earth's rotation.